Add detached payload support for JWS encoding and decoding

[fviard]

Specifications allow to have JWS with unencoded detached payloads.
This changeset adds detached payload support for encoding and decoding
functions.

For encoding, detached payload can be enabled by setting the
"is_payload_detached" arg or having the "b64=False" inside the headers.

For decoding, the detached payload content (bytes) has to be provided
with the "detached_payload" arg and "b64=False" has to be found inside
the decoded headers.

Functionnally, when this feature is used, the signature will be computed
over the raw data bytes of the payload, without being base64 encoded and
obviously, the payload will not be provided inside the generated JWS.
So, the generated JWS will look like:
base64url(header)..base64url(signature)

Relevant specifications:
RFC 7515: "JSON Web Signature (JWS)". (Annexe F)
RFC 7797: "JSON Web Signature (JWS) Unencoded Payload Option".

Usage examples:

import jwt
payload = b'{'foo':'bar','number':10}'
x_jws = jwt.api_jws.encode(payload, key,  algorithm="HS256", headers=decoded_headers, is_payload_detached=False)
print(x_jws)
# only useful to verify the signature, as, obviously, the payload is already known.
jwt.api_jws.decode(x_jws, key, algorithms=["HS256"], detached_payload=payload)

[fviard]

FYI, pushed an updated version of the PR with new tests to ensure keeping the 100% test coverage for test_api_jws.py

[auvipy]

thanks. restarted the build

[fviard]

Hi @jpadilla, is it good for you to merge?
Thanks